1. Field of the Invention
The present invention relates to a method and a system for detecting and/or locating the presence of one or more mobile telephones (hereinafter called mobiles) in a communications network, by intercepting a signal exchanged between a mobile and the network. The network is for example a cellular communications network.
The invention can be applied especially in GSM networks.
It is used for example to detect and locate emergency calls.
It can also be applied to the measurement of road traffic density by the counting of the mobiles present in a given area, for example on a roadway.
It can also be applied in all cellular communications networks with known frequency and time-related characteristics, where the signals exchanged between a mobile and a base station have waveforms with a given structure and sequences of which there is a priori knowledge (for example learning sequences, synchronisation code etc).
2. Description of the Prior Art
The prior art describes various methods and devices suited to locating emergency calls and/or to measuring traffic density by means of the GSM network.
One approach, for example, lies in modifying the mobile radio stations or the SIM (Subscriber Identity Mobile) card in order to introduce a locating function within the radio mobile unit (which is generally of the GPS mini-receiver type or the type used for field measurements on server cells and neighboring cells) and a function of sending the results of measurements from the mobile, in the form of short messages, to the network, which uses them to locate the mobile. An approach of this kind however implies the adaptation or renewal of existing GSM radio mobile units. The number of these units corresponds at the present time to several hundred million all over the world.
Another approach consists in exploiting the network signaling function on the “Abis” interface or on the A interface. Using software that works at the network level, it is possible to locate radio mobile units by implementing various methods such as the measurement of radioelectrical fields, time difference of arrival (TDOA), tracking between cells etc. The interfaces <<Abis>> and <<A>> are described, for example, in X. Lagrange, P. Gdlewski and S. Tabbane, “Réseaux GSM-DCS” (GSM-DCS Networks) Editions Hermès Science, 4th edition.
However, these approaches have certain drawbacks:                they lead to low precision and doubtful reliability of the location obtained, especially because of strong fluctuations in the propagation of the radioelectrical signals and the relatively low bit rate of the GSM system, which is 270 k bauds, for example,        they necessitate the replacement of the existing pool of mobiles (GPS receiver).        